1. Field of the Invention
The present invention relates to ceramic-coated inorganic granules having a remarkably reduced propensity to generate dust when processed, for example, into a roofing granule, and to methods for preparing same.
2. Description of Related Art
Methods have long been known for artificially coloring roofing granules (although roofing granules are discussed herein, the invention applies to any granular material having a ceramic coating). Roofing granules generally consist of crushed and screened substrate granules, in which the substrate granules are coated with a ceramic coating which may include a pigment. The coating is usually applied in the form of solution of a soluble silicate and a clay which partially or completely react together in the presence of heat to form an insolubilized ceramic coating. A chemical treatment (sometimes referred to in the art as "pickling") may be required if the temperature is too low, to ensure that a substantially water-insoluble coating is formed which is strongly adherent to the base granule. In carrying out these methods, the pigment is uniformly applied to the substrate granule surface with the soluble silicate solution.
One method of rendering the soluble silicate into an insoluble state by chemical action is by the addition of an acidic material to the soluble silicate solution, either before, during or after the application of the mixture of soluble silicate, clay, and pigment to the mineral granules. A chemical reaction between the acidic material and the alkaline portion of the silicate/clay reaction product "insolubilizes" the coating, and thus these chemicals are known in the art as insolubilization agents.
The processing of roofing granules into finished roofing panels currently requires the use of an oil coating on the granules in the process to control airborne particles of substrate and coating (particles range in size from 0.5 to about 30 micrometers) which are generated during processing. However, the addition of oil has a major disadvantage: an increase in the color difference between the color of the originally oiled granular material and the color after a given period of time. This is an especially important consideration in the manufacture of roofing granules. Oil eventually weathers off granules after a relatively short period of time and may result in a final granule color which is different than intended by the user.
The use of magnesium chloride (MgCl.sub.2) as an insolubilization agent is disclosed in the following U.S. Pat. Nos: 2,273,827; 2,591,149; 2,981,635; 2,981,636; 2,614,051; and 4,990,278.
In particular, the '149 patent discloses improving the weather resistance of colored mineral granules by providing a coating binder which comprises the reaction product of sodium silicate and a reactant such as cryolite (Na.sub.3 AlF.sub.6), and a pigment. After heating, the coated granules are treated with an insolubilizing agent which reacts with the coating to form an insoluble color bearing coating. Aluminum chloride hexahydrate is listed as the preferred insolubilizing agent; however, the specification notes that it is possible to replace aluminum chloride with "other salts of a similar nature" such as magnesium chloride, aluminum nitrate, ferric chloride, or with an acid such as hydrochloric acid or nitric acid. The only example of the patent lists the use of 22 weight percent aqueous solution of aluminum chloride hexahydrate as the insolubilizing agent, or ten pounds aluminum chloride hexahydrate per ton of granules. There is no mention of dusting or dust reduction by using these insolubilizing agents.
The '636 patent describes a similar product and process, except the pigment is added all or partially with the insolubilizing agent to the surface of the roofing granules after the soluble silicate solution has been added thereto. The advantages claimed are the reduction in the amount of pigment used and that the granules do not have the dull, hazy characteristic of granules produced by prior methods, which may be magnesium oxide if magnesium chloride is used or other insoluble hydroxide. The preferred insolubilizing agent is listed as a mixture of aluminum chloride and ammonium chloride, but either may be used alone, and magnesium chloride, aluminum sulfate, zinc chloride, hydrochloric acid, etc., may be employed less satisfactorily. The chlorides are preferably sprayed on as a 10% solution, and the examples list use of from 1.5 to 4.0 pounds chloride per ton of granules. Again, there is no mention of dusting or dust reduction by use of these insolubilizing agents.
It would be advantageous if dust levels generated during processing could be controlled to a low level without the use of oil and its attendant problems.